1. Field of the Invention
The present invention relates to a strain sensing device and ways to use the device for measuring strain. More specifically, the present invention relates to a strain sensing device including an optical fiber within a sub-assembly, wherein the sub-assembly is encased in a metallic coating which is strain coupled to the sub-assembly.
2. Description of the Related Art
Fiber optic sensors have a wide variety of applications for sensing parameters such as pressure, strain and temperature. Fiber-optic sensors possess several advantages over their electrical and electromechanical counterparts. For example, fiber-optic sensors can be made smaller, have longer lifetimes and are made from non-conducting glass, thus providing immunity from electromagnetic interferences.
In the related art, fiber optic sensors are attached to a structure of interest in such a way that strain may be measured using conventional tools. Some examples of structures of interest include, but are not limited to, casings of oil wells, bridges, buildings, steam pipes, and any other structure where strain sensing can provide predictive data on potential failure of the structure. Some techniques used to measure strain include Fiber Bragg gratings and a Brillouin Optical Time Domain Reflectometer.
Sensing with Fiber Bragg gratings includes using a sensor having a series of refractive index perturbations along an optical fiber and a light source coupled to the optical fiber. The Fiber Bragg gratings simply reflect the light traveling in the forward direction in the core of the optical fiber backwards into the core. When the sensor is strained, such as by compression or stretching of the optical fiber due to mechanical forces, or a temperature change, the spacing between the gratings varies, which correspondingly varies the arrival timing of the reflected light to the device. This effect is similar to that of an accordion, where the output note changes as the accordion is stretched and compressed. Monitoring the change in arrival timing of the reflected light can then be used to measure the strain, temperature or pressure on the optical fiber. The user may then correlate the strain on the optical fiber to determine the strain on the structure of interest.
The equipment for measuring strain is selected based on the needs of the user. Measuring the strain using Fiber Bragg gratings, for example, allows the user to measure strain in a dynamic environment with a significant improvement in speed.
The fiber optic sensor may be attached to the structure in several ways. The sensor may be attached directly to the surface of an existing structure. The sensor may also be optionally inserted into a structure either during, or after construction.
One problem with conventional strain sensors is that they are limited in their application. Related art fiber optic sensors are not sturdy enough to be employed in all but the mildest conditions. Further, even sensors having some protective coating do not survive the conditions needed to deploy these sensors successfully in environments having mechanical, chemical and pressure-related hazards. These hazards may have an affect not only on the strain sensor itself, but also the attachment method used to attach the strain sensor to the structure of interest.
One example of a harsh environment where ordinary strain sensors cannot typically survive is the application of strain sensing in oil well components. There exists a documented need in the industry for measuring the strain on the casings of oil wells. The ability to measure strain, or anticipate a potential collapse, of an oil well becomes critical to maintaining the integrity of the well, as well as saving the equipment deployed for drilling oil. However, an oil well may reach depths of well over 15,000 feet. Additionally, an oil well may be additionally submerged in water, and in some cases salt water. In this example, deploying a fiber optic sensor in such an environment would subject the sensor to issues of high earth core temperatures, high pressure due to the depth and aggressive chemical materials, which may come from the water, the ground or both.
Other examples of environments where strain sensing becomes critical is in bridges or any other concrete structure. In this environment, a strain sensor may be embedded within the concrete structure, such that the strain on the structure may be measured. Early detection of cracking of the structure's foundation allows the user to perform remedial measures early in the breakdown process, thus saving the structure from potential total and unexpected failure. A problem associated with embedding related art strain sensors in such structures relates to the integrity of the optical fiber within the sensor. Specifically that the optical fiber within the sensor is too weak to survive a potential cracking of the structure, and the integrity of the optical fiber would be compromised from even a small shift within the foundation.
The above problem is particularly prevalent in structures where the strain sensor is embedded in the concrete at the time the structure is built, thus providing the tightest fit between the structure and the sensor.
There exists a need for a strain sensing device which could survive hazardous environments such as those described above, while providing an accurate strain sensing measurement system. There further exists a need for a strain sensing device that would allow the user to anticipate potential failure of structures without compromising the integrity of the optical fiber within the strain sensing device.